1. Field of the Invention
The present invention relates to a method for preparing a heterogeneous catalyst comprised of a support material and at least one catalytically active species.
According to the present method, the catalytically active species or a reagent containing its precursor is transferred into a reaction space where it is reacted in vapour phase with the surface of the support material.
2. Description of the Related Arts
When preparing heterogeneous catalysts in the traditional manner, the catalytically active species are bonded to the surface of the support material using, e.g., impregnation, precipitation or ion exchange techniques. The initial reagents here are chemical compounds, generally salts, that are soluble in conventional solvents. The most common solvents used are water and alcohols.
The metal content of the catalysts being prepared is controlled in the impregnation technique by altering the metal compound concentration in the solution and using a certain precalculated volume of the solvent which is then used in toto to impregnate a porous support material.
This traditional method of catalyst preparation is hindered by the great number of different work phases required, whereby the risk of preparation errors increases. The catalyst preparation process is very sensitive to ambient conditions, thus necessitating very accurate control. Another disadvantage is related to the use of solvents. Namely, solvents can often react with the support material causing changes in its surface structure. Furthermore, solvents frequently contain impurities, which may affect the activity of the prepared catalyst. To avoid the disadvantages of liquid-phase techniques, several different gas-phase techniques have been developed in which the support materials are reacted with gas- or vapour-phase reagents containing the catalytically active species or its precursor. When using gas-phase techniques, the content of the metal compound in the final product is conventionally controlled by metering a certain amount of the gas into the reaction space.
Such gas-phase techniques known in the art achieve the control of the average concentration of the metal compound but fail in simultaneously achieving the control of the active species distribution on the support material. A frequently occurring phenomenon is the aggregation of the active species into clusters, so all molecules of the metal species cannot act as catalytically active points.
It is an object of the present invention to overcome the drawbacks of conventional technology and achieve a method suited to the preparation of heterogeneous catalysts having a desired content of the active species.
The invention is based on two basic ideas. Firstly, the method aims to achieve a situation in which the bonding of the gas-phase materials onto the support material surface is primarily determined by the properties of the support material surface. In the context of the present invention, this property is called the xe2x80x9csurface-bond selectivityxe2x80x9d. Namely, the goal is to achieve such process conditions in which the constituents of the reagent are selectively bonded to the bonding sites available on the support material surface, thus forming permanent surface bonds. Which bonding sites under certain conditions are available for achieving a stable end product is determined by, among other factors, the surface structure of the support material, the reaction temperature and other reaction parameters, as well as the reactivity of the reagent and its bonding energy in the reaction. The principal properties of the support material surface affecting the end result are the structural geometry of the atoms in the support material surface and their electron configuration (that is, the energy potential function of the surface).
The reaction temperature and time applied in the method, as well as other similar conditions, are determined by the support material/reagent pair. Independently of the support material and the reagent, the method according to the invention is, however, characterized in that the surface-bonding selectivity is ensured by maintaining the vapour pressure of the reagent sufficiently high and the reaction time sufficiently long to keep the amount of the reagent at least to the number of surface-bond sites available at a time.
According to the present invention, through the fulfillment of the surface-bonding selectivity requirement, a homogeneous distribution of the active species is achieved by virtue of saturating surface-bond reactions. The utilization of the saturation principle of the surface-bond reactions yields a homogeneous distribution of the active species and simultaneously controls the active species content at a saturation level which is determined by the number of surface-bond sites participating in the reaction. Consequently, the second basic idea of the invention requires that the number of those surface-bond sites which, under the predetermined conditions of set temperature, introduced reagent, and chemical structure of the support material surface, are available to form a stable surface-bond reacted product must at least essentially correspond to the desired content of the catalytically active species in the catalyst being prepared. For this purpose, according to the invention, the number of surface-bond sites is predetermined through two major variables, namely, control of the reaction temperature and/or proper selection of the reagent.
The invention combines the benefits of surface-bonding selectivity and reaction controllability. Hence, the invention makes it possible to achieve a heterogeneous catalyst whose activity even at a low content of the catalytic metal is as high as that of a catalyst of higher metal content prepared in a conventional manner. Moreover, the metal content of the end product can be accurately controlled at a predetermined level.
The definitions used in the context of the present invention are as follows:
Catalyst reagent refers to an initial reagent which is capable of being converted into gaseous form and then reacting on the support material surface so as to form a catalytically active site or a precursor necessary for generating such a site. The catalyst reagent can be any vapourizable or gaseous compound conventionally used in the preparation of heterogeneous catalysts. Thus, applicable reagent materials include, for example, elemental metals such as zinc, metal compounds such as rhenium oxides, metal halides such as halogenated chromium compounds, tungsten chlorides and oxychlorides, and metal complex compounds such as Cr(acac)3 and Mg(thd)2.
Precursor refers to such available (inactive) initial forms of the catalytically active constituent from which the active species can be obtained by means of an appropriate treatment.
Active species refers to a catalytically active component on the support material surface, whereby the active species can be in the form of, e.g.; an atom, ion, molecule, chemical compound or complex compound. Conventionally, the active species is comprised of a metal ion or atom or metal compound bonded to the support material surface.
Support material refers to a material in solid state that provides a surface of at least a relatively large area, capable of bonding the catalytically active species or its compound. The surface area of the support material determined by the BET method typically is in the range from 10 to 1000 m2/g. The support material can be comprised of an inorganic oxide such as silicon oxide (silica gel), aluminium oxide, thorium oxide, zirconium oxide, magnesium oxide, or any of their mixtures. In their inherent form, these support materials are essentially inactive as catalysts. Alternatively, support materials can be employed that inherently act as catalysts in the chemical reaction to be catalyzed. Examples of such support materials are natural and synthetic zeolites. Also inactive support materials having species of a catalytically active material bonded to their surface are considered support materials within the context of the present application. Thus, when preparing bimetal catalysts, the first catalytic species bonded to the support material surface is defined to form a part of the support surface for the second catalytic species.
Reaction space refers to the space in which the support material and the reagents are interacted with each other.
It is an object of the present invention to combine selective surface bonding with the controllability of the content of the catalytically active species. An essential property of the invention is the manner of maintaining the saturation condition during the surface-bond reaction which is characteristic of the present surface-bond selective method.
The method according to the invention comprises chiefly three phases of which the pretreatment and posttreatment phases are advantageous in some embodiments of the invention, although they are not necessary for the implementation of the basic principle of the invention.
To attain the reaction conditions favourable for the selective surface-bond reaction, all reagents necessary for the pretreatment, the bonding of the catalytically active species, and the posttreatment are introduced into the reaction space in gaseous form, typically one reagent at a time. The vapour pressure of the vapourized catalyst reagent is then maintained sufficiently high and the interaction time of the reaction with the support material surface sufficiently long so that the amount of available reagent is at least as high, or preferably in excess of the amount required to saturate the number of surface-bond sites available on the support material. The excess ratio of the reagent quantity employed to the atomic or molecular layer (known as monolayer) quantity which is necessary for filling all available bonding sites on the substrate material surface is typically in the range from 1 to 100, preferably 1 to 2. The reagent quantity corresponding to the monolayer bonding situation can be computed on the basis of the surface area of the supporting material determined with the help of, e.g., the BET method, and the molecular structure of the surface.
According to the invention, the reaction conditions are created such that the active species of the gas-phase reagent during the reaction with the support material surface can fill absolutely all or essentially all available bonding sites, whereby the saturation of the support material surface is attained at the set reaction temperature.
To prevent condensation of the reagent, the reaction temperature must not be allowed to fall essentially below the temperature necessary for vapourization of the reagent. Condensation of the reagent during its transfer to the reaction space must also be prevented. The initial reagent, its vapour temperature and the temperature used in the reaction must be selected so that decomposition of the initial reagent and possible condensation of decomposition products are prevented.
Experimental methods can be applied to determine the temperature window, or the temperature span, in which the reaction is advantageously carried out. The lower limit of such span is determined by the condensation temperature of the reagent to be vapourized and the activation energy necessary to attain a desired activation energy to establish a bond to the surface-bond site. This is because the condensation temperature of the catalyst reagent cannot be taken as the lower limit temperature for the bonding reaction if said temperature is too low for imparting to the reagent a sufficient energy to exceed the activation energy. The upper temperature limit is the lower of the following two temperatures: The decomposition temperature of the reagent or the temperature at which the constituent chemisorbed on the support material or its precursor starts to desorb in an essential amount from the advantageous bonding sites. The reagent is selected so that the activation energy necessary for chemisorption is exceeded at a temperature at which desorption from the advantageous bonding sites still remains insignificant. Because the activation and desorption energies are not generally known, the selection of the proper reagent and temperature must be performed experimentally.
The reaction between the vapour of the catalyst reagent and the support material can be carried out at elevated pressure, ambient pressure or partial vacuum. According to a preferred embodiment of the invention, the preparation takes place at a pressure ranging from 0.1 to 100 mbar. An advantage of the partial vacuum is that purity of the reaction space can be improved and the diffusion rate increased. Another preferred approach is to operate at ambient pressure. This permits the use of less complicated equipment. The preparation at ambient pressure is advantageous when the reagent under the reaction conditions has a partial pressure approximating the ambient pressure, preferably greater than 100 mbar.
The reaction time is principally affected by the diffusion of gas molecules into the pores of the support material. Diffusion of gas between the particles of the support material is rapid in comparison with the diffusion into the pores. The reaction time is selected so long as to permit a sufficiently effective interaction of the gas containing the active component of the reagent with the bonding sites of the support material and to achieve saturation of the support material surface. In the tests performed, a reaction time of 0.1 to 10 h, typically 0.5 to 2 h, was found sufficient to achieve this situation when treating support material quantities of 1 to 20 g.
In a preferred embodiment, an inert gas is conducted through a static support material column at a flow rate which remains appreciably smaller than the thermal diffusion rate of the reagent. In particular, the carrier gas flow rate is adjusted so as to be essentially equal to the diffusion rate of the reagent into the pores of the support material under the reaction conditions. This is because the saturation principle permits the use of a low flow rate that assures effective interaction between the reagent and the support material surface. Thence, the individual molecules of the reagent gas can make a plurality of impacts on the surface, which further results in an effective saturation of the bonding sites of the support material without causing a significant macroscopic oversaturation. The typical carrier gas flow rate in this embodiment is approx. 10 cm/min. The verification of the saturation condition can be performed by determination of the active species or precursor content in that part of the ready-made end product which during the reaction has been in the upper part of the support material column (that is, the carrier gas inlet end), and correspondingly, at the lower end of the column (that is, the exit end). If these two contents are equal, saturation conditions have been attained.
A pretreatment is applied to produce a predetermined number of desired bonding sites for the catalytically active species to be bonded. The pretreatment can be performed using a thermal treatment or a chemical treatment or a combination of both.
To optimize the properties of the catalyst, it can be subjected to a posttreatment if desired. This can be implemented using, e.g., a thermal treatment in which the catalyst is heated to a desired temperature which generally is slightly higher than that of the bonding reaction. When the degree of oxidization at the catalytically active point is desired to be altered, the thermal treatment is carried out in oxidizing, or alternatively, reducing conditions. The posttreatment process can also be employed to interact the prepared catalyst with a vapour, e.g., water vapour, which can affect the bonding environment of the active species or a precursor already bonded to the support material surface.
The content of the active species bonded by chemisorption is controlled according to the invention within the scope of the surface-bond selective method via the control of the saturation level, which can be implemented by varying, e.g.:
the reaction conditions (A),
the surface (B) participating in the surface-bond reaction, and
the reagent (C) introduced in gas phase.
The saturation level attained in the surface-bond reaction is determined by the combined effect of these three partial factors.
The basic control means are provided by the reaction conditions A. The most important control parameter of the reaction conditions is the reaction temperature. The variation limits of the reaction temperature and the effect of their variation is essentially dependent on the surface B and the reagent C participating in the surface-bond reaction. Each combination of B and C is related to a specific temperature window and control range of active species content.
The variation limits of the reaction temperature (temperature window) are set by the requirement of maintaining the saturation conditions. The reaction temperature offers a means for controlling the saturation level if bonding sites of different activation energies or different bonding energies for the reagent C are available on the surface B. If the surface provides bonding sites of a single type only (that is, of identical activation energy and identical bonding energy) for the reagent, the saturation level is independent of the reaction temperature within the temperature window allowable for the reaction.
If the surface has bonding sites of two different activation energy levels Ea1 and Ea2 (Ea1 less than Ea2) for the reagent, it is possible to find within the reaction""s temperature window a threshold temperature (or a temperature span of change) below which bonding sites having the activation energy level Ea1 only are filled, while when the temperature is increased above said temperature, bonds are formed to the sites of both activation energy levels Ea1 and Ea2. Then, the reaction temperature can be employed to select between two different levels of saturation.
If the surface has bonding sites of two different bonding energy levels Es1 and Es2 (Es1 less than Es2) for the reagent, it is possible to find within the temperature window of the reaction a threshold temperature (or a temperature span of change) below which bonds formed to the sites of both bonding energy levels Es1 and Es2 are retained, and above which only bonds having the bonding energy Es1 are retained. Also in this case the reaction temperature can be employed to select between two different levels of saturation.
The levels of both the activation energy Ea and the bonding energies Es can be a distributed function of energy levels, whereby the surface B provides the reagent such bonding sites whose activation energies are distributed over the range from Ea(min) to Ea(max) and whose bonding energy is distributed over the range from Es(min) to Es(max).
In the case of a distributed function of activation or bonding energy, the reaction temperature can be utilized for controlling the saturation level within the limits determined by the distributed energy functions.
On the basis of the above-discussed grounds, an advantageous embodiment of the invention is characterized in that the reaction temperature is set to a level at which the reagent introduced into the reaction space reacts so as to form a stable bond to the bonding site with only a portion of all those bonding sites which are in principle available within the temperature span confined by the upper and lower temperature limits. Advantageously, the temperature is set to a level at which the reagent introduced into the reaction space reacts with bonding sites having at least two different activation energies.
According to another preferred embodiment of the invention, the temperature is set to a level at which the reagent introduced into the reaction space reacts with bonding sites having mutually identical activation energies.
As noted in the general part of the description above, the number of surface-bond sites participating in the reaction which forms the stable end product is affected, besides by the control of the reaction temperature, also by the type of reagent selected. Consequently, according to a preferred embodiment of the invention, the reagent is selected such that the reagent introduced into the reaction space reacts with only a portion of the bonding sites available at the set reaction temperature, whereby a stable reaction product is formed with the reagent. To accomplish this, a reagent is selected, for instance, that reacts with chemically identical surface-bond sites only. One kind of these reagents are those that can react solely with the hydroxyl groups of the surface such as chromyl halides. According to another alternative embodiment, a reagent of large molecular size is selected whose molecules do not fit to bond to adjacent sites. Bonded to the surface, such a molecule will block the adjacent bonding sites of the surface thus preventing other molecules from bonding to said sites.